It is known that wobbling of pistons in internal combustion engines reduces the pressure sealing capacity of piston rings which tend to be drawn together as the piston wobbles and form clearances wherethrough gases rush from the combustion chamber to the crankcase of the engine.
For obviating piston wobbling it is necessary to minimize the clearance between the piston skirt and cylinder and thereby ensure that the piston moves inside the cylinder so that the axes of the piston skirt and cylinder would align, that is the piston skirt would move without skewing.
There is known a piston of an internal combustion engine, which provides coaxial movement of the piston skirt in the cylinder through the use of a fluoroplastic insert secured between the skirt and cylinder in a recess made in the side surface of the piston skirt (U.S., A, No. 4,158,328). The fluoroplastic insert has in cross section the form of a wedge tapering in a direction away from the piston head. The length of the recess is greater than that of the insert, and the insert is therefore capable of moving axially of the skirt from one extreme position to another. Clearances are present between the end faces of the insert and recess in such extreme positions assumed by the inserts.
However, the clearance between the cylinder and piston skirt is eliminated when the piston moves in a direction from the bottom dead center to the top dead center thus jamming the insert in the clearance, whereas the reverse travel of the piston from the top dead center to the bottom dead center causes the insert to get loose, whereby a clearance is formed between the piston skirt, insert and cylinder to result in piston wobble, faster wear and shorter service life of the piston. In addition, in the course of operation the insert tends to wear in terms of its thickness, and the clearance between the insert and the lower end face of the recess gradually diminishes. In the absence of this clearance the insert fails to function, whereby it becomes impossible to eliminate the clearance between the piston skirt and cylinder.
There is also known a piston of an internal combustion engine in which the insert acts to eliminate the clearance between the piston skirt and cylinder irrespective of the direction of movement of the piston (Deutsches Reich, A, 393,755).
The piston comprises a skirt having a piston pin hole the axis of which is perpendicular to the axis of the skirt; the side surface of the skirt having two recesses at the opposite sides of the pin hole axis with cylindrical surfaces the axes of which are parallel with the axis of the piston skirt, each such recess accommodating an insert having a cylindrical outer surface engageable with the surface of the engine cylinder having a curvature radius substantially equal to the radius of the cylinder and with the inner cylindrical surface of a curvature radius equal to the curvature radius of the recess, and two springs each disposed between the insert and piston skirt for the spring force to urge the insert to the cylinder.
In cross section the insert has a continuous thickness, since it is confined by the arcs of two concentric circles. This insert is secured in a recess with a clearance between its inner surface and the surface of the recess. The insert and recess are of equal length, which prevents the movement of the insert in the recess relative to the piston skirt axially thereof. In addition, the insert is linked with the piston skirt by a dowel to prevent it from turning about the axis of the piston skirt.
Each spring is accommodated in the respective recess between the insert and piston skirt so that its axis runs radially, whereby the radially directed spring force acts to urge the insert to the wall of the cylinder.
In view of the aforedescribed, the insert is urged by the spring exclusively in the radial direction thus eliminating the clearance between the piston skirt and cylinder and ensuring axial alignment during the travel of the piston inside the cylinder with the aim of reducing piston wobble.
However, such an arrangement of the spring necessitates a spring force exceeding the side pressure exerted on the piston. Despite the fact that the side forces exertable on the piston amount to between one fourth and one fifth of the axial (working) force imparted to the piston, they are still quite high. It is therefore necessary to use a spring of high compression force. As the piston operates and as the insert wears out, the force imparted by the spring to the insert tends to reduce to result in a shorter service life of the piston. This is accompanied by an increase in the clearance between the insert and the surface of the recess to result in disturbed coaxiality of the movement of the piston inside the cylinder and consequently in a tendency of the piston to wobble.